U.S. patent number 4,496,793 [Application Number 06/461,061] was granted by the patent office on 1985-01-29 for multi-layer metal core circuit board laminate with a controlled thermal coefficient of expansion.
This patent grant is currently assigned to General Electric Company. Invention is credited to John R. Hanson, James L. Hauser, Hendrik B. Hendriks, James F. Kilfeather, Jr..
United States Patent |
4,496,793 |
Hanson , et al. |
January 29, 1985 |
Multi-layer metal core circuit board laminate with a controlled
thermal coefficient of expansion
Abstract
The subject invention relates to a multi-layer circuit board
laminate which includes one or more stabilizing metal sheets for
reducing the thermal coefficient of expansion of the laminate. In
addition, a novel two-step fabrication method is disclosed which
permits apertures to be provided in the metal stabilizing layer and
prevents the entrapment of air in those apertures as they are
filled with epoxy. A multi-layer circuit board laminate is
disclosed having a controlled thermal coefficient of expansion
which is particularly useful in conjunction with leadless
components. The subject invention includes incorporating one or
more stabilizing metal sheets into a composite multi-layer circuit
board laminate assembly. The stabilizing metal sheets function to
significantly reduce the circuit board laminate's thermal
coefficient of expansion, thereby enabling the laminate to be used
in conjunction with leadless electronic components. In addition, a
novel method is disclosed for fabricating one embodiment of the
subject invention and includes a unique two-step lamination process
which permits apertures to be provided in the metal stabilizing
layer and allows the bonding epoxy resin to solidify within the
apertures, while simultaneously preventing the entrapment of air.
In another embodiment of the subject invention the stabilizing
layer is formed of a composite metal-dielectric laminate enabling
the layer to be provided with a non-contiguous floating type
pattern.
Inventors: |
Hanson; John R. (Richmond,
MA), Hauser; James L. (Lenox, MA), Kilfeather, Jr.; James
F. (Pittsfield, MA), Hendriks; Hendrik B. (Becket,
MA) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
26859097 |
Appl.
No.: |
06/461,061 |
Filed: |
January 26, 1983 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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162827 |
Jun 25, 1980 |
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Current U.S.
Class: |
174/259; 29/830;
428/140; 428/212; 428/901 |
Current CPC
Class: |
B32B
15/14 (20130101); H05K 1/03 (20130101); H05K
3/429 (20130101); H05K 3/4611 (20130101); H05K
2201/0355 (20130101); Y10T 428/24942 (20150115); H05K
2201/09309 (20130101); H05K 2201/09781 (20130101); Y10S
428/901 (20130101); Y10T 428/24347 (20150115); Y10T
29/49126 (20150115); H05K 2201/068 (20130101) |
Current International
Class: |
B32B
15/14 (20060101); H05K 1/03 (20060101); H05K
3/46 (20060101); H05K 3/42 (20060101); H05K
001/05 () |
Field of
Search: |
;428/139,140,212,251,416,458,596,597,607,901,902 ;29/846,830,825
;174/68.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Powell; William A.
Assistant Examiner: Bokan; Thomas
Attorney, Agent or Firm: Voss; Donald J.
Parent Case Text
This application is a continuation-in-part of Ser. No. 162,827,
filed June 25, 1980, which is now abandoned.
Claims
What is claimed is:
1. A multi-layer circuit board laminate having a controlled thermal
coefficient of expansion for use with leadless components, said
multi-layer circuit board comprising:
a plurality of planar layers bonded together in face-to-face
contacting relationship to form a complete laminate structure,
wherein at least two of said layers are thermal expansion
stabilizing layers formed of a metal having a thermal coefficient
of expansion selected to be equal to or less than the coefficient
of thermal expansion of leadless circuit board components to be
mounted on said board, said stabilizing layers being securely
bonded in face-to-face contacting relationship with at least one
other layer of said multi-layer circuit board so that said thermal
expansion stabilizing layers control the thermal coefficient of
expansion of the composite laminate structure.
2. A multi-layer circuit board according to claim 1 wherein each
said thermal expansion stabilizing layer is a metallic substance
and is located between and is bonded to two layers of dielectric
materials whereby each said metallic stabilizing layer is insulated
from any other components applied.
3. A multi-layer circuit board laminate as recited in claim 2
wherein each said thermal expansion stabilizing layer contains at
least one through hole to permit passage of electrical conductors
between components of a a circuit board using said stabilizing
layers.
4. A multi-layer circuit board laminate as recited in claim 3
wherein each said through hole contains a cylindrical annulus of
dielectric material and an electrical conductor passing axially
through the interior void of said annulus.
5. A multi-layer circuit board laminate as recited in claim 4
wherein said electrical conductor passing axially through said
annulus is a band of electro-conductive material contiguous to the
interior surface of said annulus.
6. A multi-layer metal core circuit board laminate as in claim 5
where in each said metallic thermal expansion stabilizing layer has
a thickness in the range of 0.003 to 0.015 inches.
7. A multi-layer circuit board laminate as recited in claim 1
wherein at least one said thermal expansion stabilizing layer is
copper plated on at least one surface thereof enabling said layer
to function as a voltage plane.
8. A multi-layer circuit board laminate as recited in claim 1
wherein exterior surfaces of each said thermal expansion
stabilizing layer are covered by a layer of copper oxide to promote
adhesion to the adjacent layers.
9. A multi-layer circuit board laminate as recited in claim 1
wherein at least one said thermal expansion stabilizing layer is
formed of composite laminate structure comprising a central
dielectric layer and two outer metal layers.
10. A multi-layer circuit board laminate as recited in claim 9
wherein said outer metal layers of said composite thermal expansion
stabilizing layer have a non-continguous floating pattern formed
thereon.
11. A multi-layer circuit board laminate having a controlled
thermal coefficient of expansion for use with leadless components,
said multi-layer circuit board comprising:
a plurality of planar layers disposed in face-to-face contacting
relationship and bonded together to form a composite laminate
structure, wherein at least two of said layers are formed of a
dielectric material; wherein at least another of said layers is a
thermal expansion stabilizing layer formed of a metallic substance
having a lower thermal coefficient of expansion than the dielectric
layers; wherein said thermal expansion stabilizing layer contains
at least one thru hole; and wherein at least one bonding layer is
disposed adjacent to said thermal expansion stabilizing layer with
said bonding layer consisting of at least one sheet of resin
impregnated glass cloth to facilitate the intrusion of dielectric
material into said thru hole in said thermal expansion stabilizing
layer during lamination, whereby said thermal expansion stabilizing
layer reduces the thermal coefficient of expansion of the composite
laminate structure.
Description
BACKGROUND OF THE INVENTION
In the prior art, printed circuit board laminates have been
utilized as a convenient and low cost means for mounting and
interconnecting discrete electrical components. More specifically,
printed circuit boards, formed of a dielectric substrate, are
provided with conductive metallic pathways which define electrical
connections between discrete components mounted thereon. The metal
leads of the components may be soldered to the conductive pathways
to complete the electrical connections.
The dielectric substrate used to form the printed circuit board is
generally glass fiber cloth which has been impregnated with a resin
formulation, such as epoxy or polyimide. The thermal coefficient of
expansion of the dielectric substrate is significantly greater than
that of the discrete components. The dissimilarity between the
thermal expansion coefficients has not presented an insurmountable
problem heretofore, since the flexibility of the metal leads of the
discrete components would compensate for the thermal mismatch. Even
in situations where the circuit assemblies are subjected to
frequent and great thermal changes or excursions, a manufacturer
can compensate for the thermal mismatch by providing a circuit
layout design which incorporates expansion loops in the component
leads to absorb the varied expansions and contractions of the
elements thereby preventing stress on the solder joints which is a
major cause of circuit failure.
Recently, manufacturers have developed leadless components such as
chip resistors, chip carriers or chip carriers. When the leadless
chip components, generally formed from alumina, are directly
affixed to a circuit board or to a conductive layer in a
multi-layer laminate assembly, it has been found that the
difference in thermal coefficients of expansion between the
dielectric substrate and the alumina leadless chip components, has
resulted in a high degree of circuit failures. More specifically,
the thermal coefficient of expansion for a conventional epoxy glass
laminate dielectric substrate is in the range of
15-20.times.10.sup.-6 inch per inch per degree Celsius. In
contrast, the alumina chip components have a much lower thermal
coefficient of expansion, generally about 6.times.10.sup.-6 inch
per inch per degree Celsius. Thus, when a circuit board laminate
having leadless components is subjected to high thermal excursions,
solder joints between the components and the laminate frequently
failed since there was no flexibility between the components and
the laminate to compensate for the varying amounts of expansion.
Therefore, it is apparent that it would be desirable to provide a
circuit board laminate having a thermal coefficient of expansion
which more closely matches the thermal coefficient of expansion of
the alumina chip components. Further, it would be desirable to
provide a circuit board laminate which continues to utilize glass
epoxy substrates for the circuit board construction since the
latter offers considerable cost advantages and are relatively easy
to manufacture.
Accordingly, it is an object of the subject invention to provide a
new and improved multi-layer circuit board laminate having a
controlled thermal coefficient of expansion which is substantially
the same as that of the leadless components.
It is a further object of the subject invention to provide a
multi-layer circuit board laminate which includes one or more metal
core layers for reducing the thermal coefficient of expansion of
the laminate to substantially that of the leadless components.
It is another object of the subject invention to provide an
epoxy-glass multi-layer circuit board laminate which utilizes a
metal core to reduce the thermal coefficient of expansion of a
conventional glass-epoxy printed circuit board, thereby retaining
the advantages of the latter.
It is still a further object of the subject invention to provide a
new and improved method for making a multi-layer circuit board
laminate which includes a metal layer that reduces the thermal
coefficient expansion of the laminate.
It is still another object of the subject invention to provide a
multi-layer circuit board laminate which includes a stabilizing
layer formed of a composite, metal-dielectric laminate enabling the
layer to be provided with a non-contiguous floating-type
pattern.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with the above-stated objects, a new and improved
metal circuit board laminate is disclosed having a reduced thermal
coefficient of expansion which is particularly suitable for use in
conjunction with leadless components. More specifically, a circuit
board laminate is disclosed having a plurality of planar layers
which are adhesively connected in a laminate structure via bonding
layers formed of epoxy impregnated glass cloth. In a preferred
embodiment, the circuit board laminate of the subject invention
includes an uppermost planar metallic conductive layer formed of
copper foil and a stabilizing layer formed of a metal having a
thermal coefficient of expansion less than thermal coefficient of
expansion that the composite laminate structure alone would have. A
printed circuit board layer, as used in conventional multi-layer
applications, is provided and is disposed below the metal
stabilizing layer. A lowermost planar metallic conductive layer,
similar to the uppermost layer, is disposed below the printed
circuit board layer. Both the uppermost and lowermost metallic
conductive layers are adapted to accept leadless alumina
components. Bonding sheets, formed of resin impregnated glass
cloth, are interposed between each of the layers. In accordance
with the new and improved method for producing the subject
invention initially, the uppermost planar metallic conductive layer
and the metal stabilizing layer are separately laminated to form a
partial composite structure. Thereafter, the partial composite
structure is laminated to the remaining layers to form a complete
composite laminate. This unique two-step lamination process insures
that thru holes formed in the stabilizing layer will be filled with
resin thereby preventing air from becoming trapped therein, as more
fully described hereinafter. The resulting circuit board laminate
has a thermal coefficient of expansion which is significantly
reduced and is substantially the same as the leadless components
enabling the laminate to be used with leadless components. In
another embodiment of the subject invention, the stabilizing layer
is formed of a composite metal-dielectric laminate enabling the
layer to be provided with a non-contiguous, floating type
pattern.
Further objects and advantages of the subject invention will become
apparent when taken in conjunction with the detailed description
and the accompanying drawings in which:
FIG. 1 is an exploded view of the components of the multi-layer
circuit board laminate of a first embodiment of the subject
invention.
FIG. 2 is a partial cross-sectional view of the partial composite
laminate structure which results after the first step of the new
and improved process for fabricating the subject invention.
FIG. 3 is a partial view of the completed multi-layer circuit board
laminate of the subject invention illustrating the mounting of both
conventional and leadless components thereon.
FIG. 4 is an exploded view, similar to FIG. 1, illustrating a
second embodiment of the multi-layer circuit board of the subject
invention, illustrating an alternate form of the stabilizing layer
wherein a composite metal-dielectric structure is utilized enabling
the layer to be provided with a non-contiguous pattern.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, an exploded view of the multi-layer circuit
board laminate of the subject invention is illustrated and is
referred to generally by the numeral 10. The laminate consists of
an uppermost layer 20 of a conductive metallic material, preferably
formed of a copper foil approximately 0.002 inches thick.
Underneath the metallic layer 20 a plurality of bonding sheets 22
are provided which are formed from conventional multi-laminate
board material such as resin impregnated fiberglass cloth. The
bonding sheets 22 serve as an adhesive and an electrical insulating
dielectric material between the metallic layer 20 and the metal
stabilizing layer 24 therebelow. The selection of the metal
utilized in stabilizing layer 24, which has a lower thermal
coefficient of expansion relative to the dielectric layers, is
based on the desired coefficient of expansion of the final
composite laminate and is matched thereto. Prior to lamination, the
stabilizing layer 24 is processed for profile and provided with
enlarged apertures 26 to provide clearance in the later stages of
fabrication when connective thru holes are drilled and plated. In a
preferred embodiment of the subject invention, the stabilizing
layer 24 is copper plated such that it is suitable for use as a
ground or voltage plane, in addition to its primary function of
stabilization. Both the upper and lower surfaces of the stabilizing
layer 24 may be coated with a layer of copper oxide to promote
adhesion to the bonding sheets above and below.
A conventional printed circuit board 28 is provided beneath the
stabilizing layer 24, with a second bonding layer 30 being disposed
therebetween. The printed circuit board is of conventional
multi-layered design and is preferably formed of glass cloth which
is impregnated with a polyimide or epoxy resin. The circuit board
is provided with a plurality of electrically conductive pathways 31
which define the circuit pathways to the components. It is to be
understood that while an interior circuit board layer 28 is
illustrated, it is intended that the scope of the subject include
any multi-layer laminate structure, where for example, the circuit
paths could be provided by etching the upper and/or lower metallic
layers 20 and 32.
A lowermost conductive metallic layer 32 is provided with a third
bonding layer 34 being interposed between the lowermost conductive
layer 32 and the printed circuit board 28. Both the second and
third bonding layers 30 and 34, similar to bonding layers 22, are
formed from epoxy impregnated glass cloth. The lowermost conductive
layer 32, similar to the uppermost conductive layer 20, is
preferably formed of copper foil which is 0.002 inches thick. The
upper surface of the conductive layer 20 may be pretreated with
copper oxide to promote adhesion. Both the uppermost and lowermost
conductive layers 20 and 32 are adapted to be used in conjunction
with leadless alumina components 38, as well as conventional
components 36, as illustrated in FIG. 3.
In accordance with the subject invention, a new fabrication process
is disclosed for the manufacture of the multi-layer circuit board
of the subject invention. More particularly, the prior art methods
for fabricating multi-layer boards cannot be directly adapted to
the subject invention, since the subject invention includes a metal
stabilizing layer 24 having relatively large apertures 26 which
must be completely filled with epoxy resin during the lamination
process. Further, the procedure must prevent any entrapment of air
in the apertures. Accordingly, applicants' new method includes a
two-step fabrication process wherein the uppermost metallic layer
20 and the metal stabilizing layer 24 are initially heated and
bonded to provide a partial laminate structure, and thereafter the
remaining layers are combined and heated to produce a complete
laminate structure. More specifically, in the first lamination
step, one or more bonding layers 22, formed of resin impregnated
glass cloth, are interposed between the upper metallic layer 20 and
the metal stabilizing layer 24. A relatively large amount of epoxy
resin is required to completely fill all the apertures 26 in the
metal plane. The metallic and stabilizing layers 20 and 24 are then
laminated such that the resin in the bonding sheets 22 fill up
apertures 26, thereby forming a partial laminate structure. This
separate, initial lamination step permits the resin to flow freely
into apertures 26 without the risk of entrapping air therein, as
illustrated in FIG. 2.
In the second lamination step, the remaining layers are united with
the partial lamination structure and laminated to produce a
complete laminate structure, as illustrated in FIG. 3. Conventional
methods are used to complete the final fabrication such as the
drilling and plating of thru holes which facilitate the formation
of the electrical connections. Thereafter, the metallic layers 20,
32 may be etched and provided with termination pads for leadless
components in accordance with standard multi-layer circuit board
fabrication.
Test results of the multi-layer circuit board laminate of the
subject invention, as illustrated in FIGS. 1-3, utilizing metal
stabilizing layers having thicknesses ranging from 0.005 to 0.015
inches, produced an average thermal coefficient of expansion of
approximately 8.9.times.10.sup.-6 inch per inch per degree Celsius
over a range of -55 to 125 degrees Celsius. The multi-layer circuit
board laminates, assembled with leadless chip carrier components,
withstood more than 400 thermal stress cycles per Mil Std 202
(thermal cycling of assembled circuits between -55 and 125 degrees
Celsius) with no failures in 2000 solder joints.
Referring to FIG. 4, there is illustrated a second embodiment of
the subject invention, which, similar to the first embodiment,
includes a plurality of layers which may be laminated to form a
multi-layer metal core circuit board having a controlled thermal
coefficient of expansion. More particularly, a circuit board 110
includes upper and lower copper foil layers 120 and 132 having a
thickness of approximately 0.002 inch. A circuit board layer 128
may be provided which is formed of a dielectric substrate, and
includes electrically conductive pathways 131 formed thereon. A
plurality of layers of bonding sheets 122, 124 and 134 are
interposed between the other layers to act as a dielectric and to
supply the epoxy resin necessary to secure the laminate
structure.
In the second embodiment of the subject invention, a composite
stabilizing layer 140, having low thermal coefficient of expansion
characteristics, is disclosed. More specifically, stabilizing layer
140 consists of a central dielectric substrate 142 and two outer
metallic core layers 144 and 146, laminated together to form a
composite structure. The dielectric substrate layer 142 may be a
resin impregnated glass cloth, while the metal core layers 144 and
146, each having a thickness in the range of 0.003 to 0.010 inches,
are selected to match the desired expansion characteristics of the
final circuit board laminate.
The stabilizing layer 140 of the second embodiment of the subject
invention provides certain advantages over a single layer metal
core stabilizing layer. More specifically, and as illustrated in
FIG. 4, the configuration of the pattern formed in the metal core
144 does not have to be contiguous with the core panel, since the
entire core is supported by a dielectric substrate. Stated
differently, the stabilizing layer 140 may be provided with a
"floating type" core pattern which may include, for example,
portions 148 that are non-continguous with the remainder of the
core 144. Another advantage of the composite stabilizing layer 140
is that during the lamination process the relatively large open
areas 150 of the core layers are more easily filled with resin.
Thus, the danger of air being entrapped in the open areas 150 is
significantly reduced, enabling the layers to be laminated in a
single step. As in the first embodiment of the subject invention,
stabilizing layer 140 may be plated with an electroconductive
metal, enabling the layer to act as a ground or voltage plane.
In summary, there is provided a new and improved multi-layer
circuit board laminate and method of making the same having a
reduced thermal coefficient of expansion which substantially
matches that of the leadless components to be attached thereto.
More particularly, a multi-layer circuit board laminate is
disclosed having one or more metal stabilizing layers incorporated
therein. The circuit board laminate includes uppermost and
lowermost layers which are adapted to accept leadless components.
Sandwiched between the metallic layers is a conventional printed
circuit board layer, and one or more stabilizing layers formed from
a metal having a thermal coefficient of expansion which
approximates that of the leadless components to be attached
thereto. A plurality of bonding sheets, formed of resin impregnated
glass cloth, are interposed between each of the above-stated layers
to act as an adhesive. In the novel fabrication process, the
uppermost metallic layer is initially laminated with the metal
stabilizing layer such that the resin in the bonding sheets can
flow freely into the apertures provided in the metal stabilizing
layer without the risk of entrapping air therein. A partial
laminate structure is formed which is then laminated with the
remaining layers to form a complete composite laminate structure.
The latter structure may be finished according to conventional
fabrication methods. The subject invention functions to reduce the
thermal coefficient of expansion of the circuit board laminate so
that it may be used in conjunction with alumina components which
have thermal coefficient of expansion characteristics which are
substantially the same. Further, the use of the metal stabilizing
plane provides excellent surface conditions for multi-layer board
laminates and in addition, can perform as a ground or voltage
plane.
Although the subject invention has been described by reference to
preferred embodiments, it will be apparent that many other
modifications could be devised by those skilled in the art that
would fall within the spirit and scope of the present invention as
defined by the appended claims.
* * * * *